A gas turbine engine 10 is shown in FIG. 1 and comprises an air intake 12 and a propulsive fan 14 that generates two airflows A and B. The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, nozzle guide vanes 23, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28. A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32.
As well as flowing directly to the combustor 20 for combustion with fuel, a portion of the air flow A flowing through intermediate and high pressure compressors 16, 18 is diverted and used to cool the engine components such as turbine blades 22, 24, 26 and nozzle guide vanes 23 (NGVs). However, as air flows through the intermediate and high pressure compressors 16, 18, the compression of the air causes the temperature of the air to rise to around 700° C. or higher. This reduces the cooling capacity of a given mass of air, since the air is at a higher temperature before it is delivered to the turbine components 22, 24, 26 and NGVs 23. As a result, more air is required to cool the turbines to a required temperature, resulting in reduced efficiency of the engine 10.
It is therefore desirable to cool engine cooling air after it has been compressed by the compressors 16, 18 before it is used to cool engine components downstream, such as components 22, 24, 26, 23. One prior system for providing cooled engine cooling air comprises a fuel air heat exchanger. Relatively hot engine cooling air from the compressors 16, 18 and relatively cool liquid hydrocarbon fuel supplied from a fuel tank (not shown) is passed through a heat exchanger matrix such that the engine cooling air is in thermal contact with the fuel. Heat from the engine cooling air is transferred to the fuel by the heat exchanger as the air and the fuel flow through the matrix. The fuel then flows to the combustor 20 where it is mixed with the remainder of the air from the compressors 16, 18 and burnt in a conventional manner.
However, the useable cooling capacity of hydrocarbon fuel is limited by a “critical temperature” above which the fuel may lose its required properties. In the absence of dissolved oxygen, the fuel will decompose (i.e. undergo pyrolysis) at temperatures above a decomposition temperature of the fuel. The decomposition temperature of the fuel is specific to the particular fuel. In the case of jet A1 hydrocarbon fuel, the decomposition temperature is in the region of 371° C. Furthermore, in the presence of dissolved oxygen, insoluble compounds known as “coke” will be formed where the fuel is heated to a temperature above an oxidation temperature. The oxidation temperature will again generally be dependent on the particular fuel, and on the oxidation state of the fuel, i.e. the amount of dissolved oxygen present in the fuel. In jet A1 hydrocarbon fuel having a dissolved oxygen level typical in aviation applications, coke may begin to form in fuel that is heated to a temperature above 150° C. Coke deposits can cause degradation of fuel delivery performance by blocking or partially blocking fuel lines and/or injectors.
One proposed solution to the above problem is to deoxygenate the fuel prior to delivery of the fuel to the fuel air heat exchanger using a fuel deoxygenator, either by deoxygenating the fuel prior to fuelling the aircraft, or by deoxygenating the fuel from the fuel tank prior to passing the fuel through the heat exchanger (as outlined for example in EP 1559883). The coking temperature of the fuel is thus increased, and the fuel can then be heated to a higher temperature before coking occurs, thereby permitting more heat to be transferred from the air to the fuel.
However, in modern gas turbine engines, the temperature of the air flow may nevertheless be sufficient to raise the temperature of the deoxygenated fuel in the fuel line to a temperature above the critical temperature (i.e. either the coking temperature or the pyrolysis temperature). It has been found that the fuel may also become partially re-oxygenated (i.e. the amount of dissolved oxygen will increase) after a period of time.
The present invention seeks to address some or all of the above problems.